A typical mass flow controller (MFC) is a closed-loop device that sets, measures, and controls the flow of a gas in industrial processes such as thermal and dry etching among other processes. An important part of an MFC is a sensor that measures the mass flow rate of the gas flowing through the device. Typically, a closed-loop control system of the MFC compares an output signal from the sensor with a predetermined set point and adjusts a control valve to maintain the mass flow rate of the gas at the predetermined set point.
A closed-loop control algorithm, if properly tuned, can be used to adjust a flow of a fluid in response to changes in fluid flow conditions that cause deviations away from a specified fluid flow set point. Changes in fluid flow conditions are often caused by variations in, for example, pressure, temperature, etc. Deviations away from the specified fluid flow set point caused by these variations are detected and corrected for based on measurements (e.g., feedback signal) generated by a sensing device (e.g., flow sensor measurements from a flow sensor) within a feedback loop of the closed-loop control algorithm.
When fluid flow conditions, however, change rapidly as a result of, for example, rapid pressure changes, sensing devices used by the feedback loop may saturate or produce unreliable feedback signals. If a flow controller, for example, uses these saturated and/or unreliable feedback signals within the closed-loop control algorithm, the flow controller may not deliver the fluid according to the specified fluid flow set point. The flow controller may, for example, over-compensate or under-compensate for changes in fluid flow conditions based on the unreliable feedback signals.
Another mode of operation where closed-loop systems do not perform well is when the valve is relatively far from a required position. For example, when an MFC is at a zero set point (zero valve position), and then is given a non-zero set point, it takes a relatively long time for the valve to move from the zero position to a position where noticeable flow appears and the closed-loop algorithm starts working properly. This results in a long response delay and poor performance of the MFC.
Open-loop systems have been utilized within MFCs to improve control over process gases when closed-loop systems do not perform well. In these systems, valve characterization data obtained in connection with a calibration gas (e.g., nitrogen) has been utilized to control the position of a valve of the MFC in an open-loop mode of operation. But the valve characteristics for different process gases may be very different than the calibration gas; thus if these typical MFCs are running a process that is different than the calibration gas the performance of the MFC may degrade significantly.
Accordingly, a need exists for a method and/or apparatus to provide new and innovative features that address the shortfalls of present closed-loop and open-loop methodologies.